The use of a gas cluster ion beam (GCIB) for etching, cleaning, and smoothing surfaces is known in the art. GCIBs have also been employed for assisting the deposition of films from vaporized carbonaceous materials. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters may consist of aggregates of from a few to several thousand molecules or more that are loosely bound to form a cluster. The clusters can be ionized by electron bombardment, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges of qe (where e is the magnitude of the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). The larger sized cluster ions are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently, the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes gas cluster ions effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage, which is characteristic of conventional ion beam processing.
Presently available cluster ion sources produce cluster ions having a wide distribution of sizes, N, up to N of several thousand (where N=the number of molecules in each cluster). Clusters of atoms can be formed by the condensation of individual gas atoms (or molecules) during the adiabatic expansion of high-pressure gas from a nozzle into a vacuum. A skimmer with a small aperture strips divergent streams from the core of this expanding gas flow to produce a collimated beam of clusters. Neutral clusters of various sizes are produced and held together by weak interatomic forces known as Van der Waals forces. This method has been used to produce beams of clusters from a variety of gases such as argon, oxygen, nitrogen, nitrogen trifluoride, sulfur hexafluoride, diborane, boron trifluoride, and germane.
Several emerging applications for GCIB processing of workpieces on an industrial scale are in the semiconductor field. Although GCIB processing of workpieces is done using a wide variety of gas cluster source gases, many of which are inert gases, in many semiconductor processing applications it is desirable to use reactive source gases in the formation of GCIBs, sometimes in combination or mixture with inert or noble gases. When using a combination of source gases, all source gases to be delivered from the source canister, cylinder, or system are mixed at a single high pressure for entry into the nozzle. Compressing a low-pressure source to a pressure equal to a high-pressure source, such as with a piston, rotary vane, roots blower, or scroll type mechanical pump, may lead to source problems such as fouling or plugging due to nucleation and condensation of the low-pressure source during mechanical compression or after compression but before entry to the nozzle.